Question
I found the compost (fine bark) of the cymbidium is strange.  it has while fir stuff grow in the surface of the compost, and when I went deeper into the compost, I found that almost half pot of the compost is like that. but the root is OK, only I found the leaves has big dark spot, and the leaves turn yellow.

I found another pot with the same thing, and the water seems not get into the compost as easy as others.

What is this, should I throw all the orchids with this kind of compost away ? they are giving me spikes now.


Answer
The white growth in the pot is a type of mold.  It suggests that the potting mix is staying too wet and does not have enough air flowing through it.  I assume the plants have not been repotted recently and need to be using a coarser orchid potting mix.  Over time, the orchid potting mix decays (just as any wood would)leaving a more soil-like mixture which holds water longer and reduces air flow to the roots.

What to do?  Since your plants are coming into spike, it is risky to report at this time.  You could try this on a "test" plant to see if it retards spike growth.  Get a fresh bag of orchid potting mix from a garden center and soak it well overnight and then let it drain.  Then, unpot one of the cymbidiums, being careful not to damage the flower spike.  Wash off all of the roots and remove any that are "mushy" before repotting. Place the plant back into your growing area and see if the spike continues to elongate. After a week, feed feed with a high phosphorous fertilizer at the rate of 1/2 tsp per gallon of water and continue this once weekly.  If, after a week or two, the spike seems to behave normally, then do the same with your other cymbidiums.  If the spike growth is affected, then wait to repot the others until after blooming.

Remove all black spots by cutting off the leaf below the spot.  Water or feed plants as early in the morning as possible to prevent further leaf spotting.